The spinal column, better known as the backbone or spine, is a highly complex system comprising connective tissues for bones to offer a support part for the human body and protect sensitive spinal cord and nerve roots. The spine includes a series of vertebrae stacked one on top of the other, and a pedicle of each vertebra includes an inner part composed of relatively soft cancellous bones and an outer part composed of stronger cortical bones.
For treatment of diseases related to the spine, an indirect treatment method using physical therapy and a direct method of correcting or fixing the spine by mounting an alternative fixation device to an injured pedicle are typically conducted. Physical therapy is often used for slight spinal disorders, however, in case of serious diseases of cervical vertebra, thoracic vertebra, lumbar vertebra, sacrum, intervertebral disk or the like, which form the spine, treatment using a spinal fixation device is normally performed.
As one of the direct treatment methods described above, spinal fixation using a spinal fixation device refers to a technique to fix the spine by using an orthopedic rod, which approximately extends in parallel to the spine and is typically called a ‘spine rod,’ and may be accomplished by exposing the back portion of the spine and inserting a spine fixation screw into the pedicle of a desired vertebra.
Generally, two spine fixation screws are mounted on each vertebra and function as a fixation point for the spine rod. Therefore, an arrangement of the spine rods allows the spine to be fixed in a more advantageous shape. Such a spine fixation screw consists of a screw rod to be inserted and fixed in the pedicle and a screw head integrally coupled with the screw rod.
A conventional and traditional spine fixation screw does not have any structural difference between a part of a screw thread in the screw rod to be inserted and fixed in the pedicle, which is embedded in a cancellous bone, and another part thereof, which is embedded in a cortical bone, thus causing a problem that the spin fixation screw could not be strongly coupled with bone tissues.
In other words, since the cancellous bone is a soft bone tissue region while the cortical bone is a region composed of relatively rigid and compact outer bone, it is important that the spin fixation screw has a structure including a screw rod and a screw thread exactly matching with each region. If not, a coupling of a screw member to the region may be loosened by external force, which in turn does not offer sufficient coupling force and resistance required to fix the spine. As a result, there is a problem of difficulties in fixing an injured spine.
In order to solve the above problem, a variety of conventional methods have been invented, including, for example, U.S. Pat. No. 9,247,976 B2 (entitled “Multiple-thread bone screw and method thereof”) and US Patent Laid-Open Publication No. US 2013/0053901 A1 (entitled “Bone anchor”).
U.S. Pat. No. 9,247,976 B2 describes that a screw body fixed to a cancellous bone 100 forms a single-strand screw pattern while another screw body fixed to a cortical bone 102 forms a double-strand screw pattern (see FIG. 1). US Patent Laid-Open Publication No. 2013/0053901 A1 describes that a distal screw thread connecting part 204, second screw thread connecting part 52 and third screw thread connecting part 54 are all double screw thread types while a proximal screw thread connecting part 202 is a quadruple screw thread type (see FIG. 2).
As such, according to the disclosures of U.S. Pat. No. 9,247,976 B2 and US Patent Laid-Open Publication No. 2013/0053901 A1, a screw thread of the screw body fixed to the cortical bone is more densely formed, compared to a screw thread of the screw body fixed to the cancellous bone.